Combustion system with aerodynamically variable geometry



. septy s; 19 70 I W.V D.'- BRYCE COMBUSTION SYSTEM-WITHAEBODYNIAIMICALLYY VARIABLE GEOMETRY Filed July 31, 1968 2 SheetS-Sheetl r s M Ma R M Ma HA u. W

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. Filed July 31, 1968 w. D. BRYC-E' 3,527,052 1C0MBU'STI0N SYSTEM WITHAERCDYNAMICALLY VARIABLE GEOMETRY 2 Sheets-Sheet 2 FIG. 2.

Inventor B wuu/m usnyce mm, mm, mus: 'nosus/c i ttorntys United StatesPatent 3,527,052 COMBUSTION SYSTEM WITH AERODYNAM- ICALLY VARIABLEGEOMETRY William Dean Bryce, Farnham, Surrey, England, as-

signor to Minister of Technology in Her Britannic Majestys Government ofthe United Kingdom of Great Britain and Northern Ireland, London,England Filed July 31, 1968, Ser. No. 748,998 Claims priority,application Great Britain, Aug. 10, 1967, 36,856/ 67 Int. Cl. F02c 7/00US. Cl. 6039.65 12 Claims ABSTRACT OF THE DISCLOSURE Suction is appliedto the surfaces of passages by which air enters a combustion chamber, tocontrol the pattern of air flow in order to vary the fuel to air ratioand other conditions in the primary combustion zone. In one embodiment,a baflle in the air inlet to a combustor surrounds a spray atomiser toform an annular duct by which air passes to the primary zone. Furtherair passes round the outside of the baflle which is provided withsuction slots in its surface. By varying the amount of suction, thedirection of the further air can be altered and the volume flowing tothe primary and secondary zones varied accordingly.

In another embodiment, the end wall of a flame tube surrounding the airinlet to the primary combustion zone is a hollow structure having anopening facing upstream of flow through the combustion chamber. Slotsare provided in the downstream face of the end wall and around itsperiphery whereby air entering the opening can be directed along theface and the flame tube walls for cooling. Adjacent to the peripheralslot is an annular tube having rows of holes in its surface so thatsuction applied to the tube will act on the cooling air issuing from theslot to reduce its velocity and change its direction. The cooling streamnormally passes over secondary air inlet holes in the flame tube wallsand directs the secondary air in a generally downstream direction togive an extended recirculation pattern. Diverting the cooling stream hasthe effect of varying the inlet angle of the secondary air and thusreduce the length of the recirculation pattern.

The invention relates to combustion apparatus as may be used in gasturbine plant.

In conventional gas turbine plant, a compressor discharges air into acombustion system wherein fuel is introduced into the flow and burned,the combustion gases so produced being led to a turbine.

Generally, the compressor supplies more air than is necessary forcomplete combustion of the fuel and it is usual to divide the airflowing to the combustion system into two or more streams One of whichgoes to initiate and support combustion while another is used to dilutethe hot combustion products to reduce their temperature to a valueacceptable to the turbine.

In gas turbine engines for aircraft, combustion systems are required tooperate over a wide range of conditions which involve differing ratiosin the mass flows of the combustion and dilution air streams. It istherefore desirable to have means whereby the distribution of air beweenthese streams may be varied at will and it has been previously proposedto inject a supply of controlling air under pressure into a compressormain stream to divert flow between combustion and dilution streams.

According to the invention, a combustion system in which fuel may beburned continuously in a stream of air includes a combustion chamber,means associated p CC with the combustion chamber for dividing airflowing thereto into at least two streams and means operable to vary thedistribution of flow between the streams wherein suction is applied to asurface forming a part of the combustion chamber in such manner as tovary the direction of air flowing over the surface.

Preferably the suction is applied through openings in said surface toact on the boundary layer of the air flowing over the surface.

Alternative embodiments of the invention will now be described by way ofexample with reference to the accompanying diagrammatic drawings ofwhich:

FIG. 1 is an axial section through a combustion chamber, and

FIG. 2 is a similar view of another combustion chamher.

The combustion system illustrated in FIG. 1 follows conventionalpractice, comprising a combustion chamber 1 containing a flame tube 2,which encloses the actual combustion space, and a fuel injector 3. Thecombustion chamber is generally cylindrical and tapers at both ends toform respectively an inlet duct 4 from a compressor (not shown) and anoutlet 5 intended to be connected to a turbine inlet. The flame tube,also generally cylindrical, is mounted co-axially within the combustionchamber with one end tapering inwardly to meet the wall of thecombustion chamber at the outlet 5 and the other end facing upstreamtowards the inlet duct 4. An annular space 6 which is closed at itsdownstream end and open to flow from the inlet duct at its upstream endis formed between the combustion chamber and the flame tube.

The fuel injector 3 is located on the axis of the combustion chamberadjacent to the inlet end of the flame tube so as to direct a conicalspray of fuel into the flame tube as shown by the dotted lines, the fuelbeing supplied to the injector by a pipe 7. The fuel injector is carriedby support struts 8 extending inwardly from the wall of the combustionchamber across the flow path from the inlet duct 4. The support strutsalso carry an annular flow divider 9 which surrounds the fuel injectorand whereby a part of the incoming flow is directed round the outside ofthe divider towards the annular space 6, the remainder passing into theflame tube by way of an inlet passage 10 between the flow divider andthe body of the fuel injector as primary combustion air. Air from theannular space 6 may pass to the interior of the flame tube by way of arow of small diameter holes 11 spaced circumferentially around the flametube about one-third of the way along its length and by a row of largerdiameter holes 12 similarly spaced about two-thirds along to constitutesecondary combustion and dilution air respectively.

The flow divider 9 comprises a ring of approximately D-section, thestraight side forming the inner periphery and the rounded side beingslightly flattened in the upstream direction. The downstream face isspaced from the flame tube by an annular gap 13. Two concentric annularslots 14, 15 pass through the downstream face of the flow divider (i.e.,that which faces towards the flame tube) and are connected by internalpassages 16, 17 respectively to suction tubes 18, 19.

Normal flow through the combustion chamber when in operation is shown bythe solid arrows. Air from the compressor flows in through the inletduct 4, that which forms the primary combustion air passing through theinlet passage 10 to atomise the fuel spray from the injector. Theresultant mixture is ignited by conventional means (not shown) to burnin a combustion zone situated towards the upstream end of the flametube.

Air passing over the upstream face of the flow divider is directed tothe annular space 6 to flow through the holes 11 and 12 and mix with thehot gases in the flame tube to complete the combustion process andreduce their temperatures before they pass to the outlet 5.

Some air will flow round the downstream face of the flow divider toenter the flame tube through the annular gap 13 but this will notnormally be a significant quantity.

However, suction applied to the passages 16, 17 in the flow divider willact through the annular slots 14, to draw off the boundary layer in thisregion. As a result the rate of air flow through the annular gap 13 willincrease and larger quantities of air will be directed to the combustionzone as shown by the dotted arrows. This air thus by-passes the annularspace 6 and the air flow through the holes 11 and 12 is correspondinglyreduced to effect a change in the combustion pattern.

The variation in flow may be controlled by the degree of suction appliedand by the use of only one of the annular slots at a time. The lastfeature may also be utilised to change the direction of flow into theflame tube through the annular gap 13 where variations in flame lengthare involved. (Differential suction between the slots may also be usedin this connection.)

The combustion system shown in FIG. 2 is generally similar to that ofthe previous embodiment and the same reference numerals are used todenote corresponding components. As before, incoming air from an inletduct 4 is directed in part to an annular space 6 surrounding a flametube 2 and another part of the air passes through an inlet passage 10surrounding a fuel injector 3 into the flame tube at its upstream end.Two rows of holes for secondary combustion air are spacedcircumferentially around the flame tube adjacent to its upstream end anda further row of smaller holes 31 between these and the large holes 12for dilution air admit intermediate air.

The inlet passage 10 is defined by an annular baffle 21 spacedintermediate between the fuel injector and the flame tube and curvinggenerally outwardly at its downstream end to extend transverselypartially across the flame tube near its upstream end. A further baflie22 extends transversely between the curved end of the baffle 21 and thewall of the flame tube. The baffles 21, 22 together with the upstreamend of the flame tube bound an annular channel 23 having an upstreamfacing opening through which air can enter from the inlet duct 4. Theopposite ends of the baflle 22 overlay the adjacent end of the baffle 21and the wall of the flame tube to form cooling passages 24, 25respectively from which air may be directed over the surfaces of thebaffle 22 and the flame tube wall from the annular channel 23. Air fromthe cooling passage 25 enters the flame tube immediately upstream of thesecondary air entry holes and its velocity is normally such that thesecondary air is diverted in a downstream direction as shown by thearrows B near the top of the figure. The secondary air combines withcombustion gases in the flame tube and with intermediate and dilutionair flowing from the annular space 6 through the holes 31 and 12 to forman extended recirculation pattern as indicated on the upper half of thefigure.

A circular pipe 26 is mounted within the flame tube in close proximityto the outlet from the passage 25. A series of holes 27 extendcircumferentially through the surface of the pipe on its downstream sideand the interior of the pipe is connected to a suction tube 28. Theeffect of applying suction will be to draw off some of the cooling airentering the flame tube through the passage 25 causing it to flowinwardly towards the centre of the flame tube. Thus it will notinfluence the secondary air so strongly and this too will tend to flowtowards the centre of the flame tube as indicated by the arrows Ctowards the bottom of the figure with consequent reduction in therecirculation pattern.

In alternative constructions, suction can be applied through holes,meshes, porous media or other openings which permit passage of airthrough surfaces forming airflow boundaries or which act to divide anairflow, for example, the surfaces of flow guide vanes at a combustionchamber inlet.

Air pressure within a combustion system is usually higher than exists atother parts of a gas turbine engine. Suction may thus be readilyachieved by providing connections from those locations where it is to beapplied to another part of an engine or to the atmosphere. Valves may beemployed to control the magnitude or application of the suction, suchvalves being controlled by external means or automatically by signalsfrom sensors responding to changes in operating conditions (e.g.,pressure, temperature, density, speed, flow in the engine or fueldelivery rates).

Other than acting directly on airflow to induce changes therein, suctionmay be employed to move a surface to cause redistribution of flow. Onesuch arrangement is to apply suction to one side of a surface which ispivoted at or near one end and subjected to aerodynamic loading on itsother side. Some form of servo-mechanism operated or controlled by theapplication of suction can also be envisaged.

The invention is equally applicable to any of the forms of combustionchamber, such as annular, turbo-annular and can-type (as shown)configurations, used with gas turbine engines.

I claim:

1. A combustion system for the continuous combustion of fuel in a streamof air, comprising a combustion chamber having an air inlet, part of thecombustion chamber constituting a combustion zone, means arranged todirect a proportion of inlet airflow to the combustion zone,

duct means within the combustion chamber arranged to lead a significantpart of other inlet airflow downstream of the combustion zone fordilution purposes, and

a surface forming a part of the combustion chamber and selectivelyconnected to a source of suction in such manner that application ofsuction will induce an airflow over the surface so directed as toincrease the quantity of air passing to the combustion zone duringoperation of the combustion system.

2. A combustion system according to claim 1 comprising at least oneopening in said surface and means connected to apply suction to saidopening.

3. A combustion system according to claim 1 further comprising a flametube within the combustion chamber, holes formed in the wall of theflame tube, and a baffle whereby an airstream is directed to deflectfurther air entering the flame tube through the said holes, theaforesaid surface being disposed in the airstream in such manner thatsuction when applied will act on the boundary layer of air flowing overthe surface to vary the direction of the said airstream.

4. A combustion system according to claim 1 including a flow dividerdisposed in the air inlet and having at least one opening extendingthrough the surface of the flow divider, each such opening beingconnected to a source of suction.

5. A combustion system according to claim 4 in which the flow divider isof annular configuration.

6. A combustion system according to claim 5 further comprising a flametube and a fuel injector arranged to direct a spray of fuel into theflame tube, the flow divider being disposed about the fuel injector.

7. A combustion system according to claim 5 having openings extendingcircumferentially around the flow divider.

8. A combustion system according to claim 5 having at least twoconcentric annular slots extending through the surface of the flowdivider.

9. A combustion sytem according to claim 8 in which each slot isseparately connected to a source of suction.

10. A combustion system according to claim 1 further comprising a flametube within the combustion chamber, a fuel injector arranged to direct aspray of fuel into the flame tube, and an annular flow divider situatedupstream of the flame tube and surrounding the fuel injector, whereinthe flow divider includes at least two concentric annular slotsextending through its surface and each slot is separately connected to asource of suction, the slots being arranged so that the application ofsuction will draw off the boundary layer of air flowing over the flowdivider surface to vary the relative distribution of air flowing intoand around the flame tube.

11. A combustion system according to claim 3 in which the baflle and thewall of the flame tube together define a passage whereby air may bedirected over the flame tube wall and further comprising a perforatedsurface disposed adjacent to the outlet of the passage and connected toa source of suction.

References Cited UNITED STATES PATENTS 2,807,933 10/1957 Martin 6039.39XR 2,960,823 11/1960 Fox 60-3929 XR CARLTON R. CROYLE, Primary Examiner

